and is the aperture radiance that gives a signal/noise ratio of unity. 
The factors governing the radiometric resolution of electro-optical 
sensor systems are considered in detail by Slater (1974). 
Spatial resolution for electro-optical systems is defined by NASA 
in terms of instantaneous field of view (IFOV) or effective instantaneous 
field of view (EIFOV). IFOV relates to the earth area subtended by a 
sensor detector from a nominal altitude and is generally specified in 
milliradians or the equivalent ground dimension. If referred to an MTF, 
the IFOV corresponds to the cutoff spatial frequency, Ve (Fig. 9). 
EIFOV, by contrast, is taken as the equivalent ground width of a 
half cycle at the spatial frequency defined by the 50 percent modulation 
point on the system MTF (NASA, 1973). As with photographic systems, the e 
total MTF is the cascade of the component MTF's, i.e.: 
MTF(v) = MTF(v) x MTF(v) x MTF(v) 
SyS optics detector image smear 
For the systems being discussed detector size limits performance (as 
does the film in many photogrammetric camera systems), and the MTF for 
a square detector is given by a sinc function (i.e., (sin 7Dv)/T7Dv, 
where D is the detector dimension parallel to direction of interest). 
In Fig. 9, the MTF for the LANDSAT MSS detectors which have IFOV's of 
79 x 79 m is plotted for the 70 mm image format scale of 1:3,369,000. 
The cutoff spatial frequency, v_, is 42.6 cy/mm, whereas the spatial fre- 
quency for 50 percent modulatiofi occurs at v /1.67. Thus, the MSS EIFOV 
corresponds to a spatial frequency of 25.5 1pr/mm, or a ground dimension 
of 66 m (i.e., 1/51 mm x 3,369,000). By comparison, the half-cycle for 
a ground dimension of 79 m (IFOV) occurs at a spatial frequency of v /2 - 
21.3 cy/mm, which has a modulation of 64 percent. The EIFOV concept is 
based on the assumption that other system components will degrade the 
detector MTF to approximately 50 percent response at the spatial fre- e 
quency equivalent to v./2. The 16 percent difference in the ground di- 
mensions of the IFOV (79 m) and the detector EIFOV (66 m) are relatively 
insignificant. For example, referring to Fig. 10, a detector MTF approximates 
the MSS system MTF obtained from analyses of band 5 images using EGA, 
line-spread function (LSF) and scale matching analysis (SMA) techniques. 
Thus, the detector MTF can be employed to approximate system perform- 
ance (Welch, 1974a, Schowengerdt, Antos and Slater, 1974; Schowen- 
gerdt, 1976). 
Although somewhat simplified, these measures of performance can be 
.used to evaluate planned earth satellite sensor systems. For example, 
LANDSAT-C, scheduled for 1977, will employ a MSS and two RBV's similar 
to those of LANDSAT-1 and -2. However, a thermal channel (10.4-12.6 um) 
with an IFOV equivalent to 238 m will be added as a 5th band for the MSS; 
and the focal length of the RBV's (both operating in the .5-.75 um spectral 
band) will be increased from 125 mm to 236 mm. The LANDSAT Follow-On 
mission (71980) will include an improved multispectral scanner, the The- 
matic Mapper, operating in five discrete spectral bands from 0.45-1.75 um, 
plus a sixth thermal infrared band of 10.4-12.5 um.  IFOV's of 30 m are e 
planned for bands 1-5 and 120 m for band 6.